The present invention relates to a so-called active-matrix-driven liquid crystal display device which is driven by TFTs (Thin Film Transistors) and, more particularly, to a small-sized liquid crystal display device to be used for projection systems.
FIG. 13 shows an example of the structure of a driving system for a small-sized active-matrix-driven liquid crystal display (LCD) device which has conventionally been used in a projection system. FIG. 17A schematically shows an example of the drive method. Conventionally, because small-sized active-matrix-driven LCD devices used for projection systems require an extremely small pitch of connections of a driver LSI (Large Scale Integrated Circuit), a so-called driver-monolithic LCD device is generally used, which integrally incorporates a driver by using polysilicon TFTs.
As to the driving operation, as shown in FIG. 13, a video data signal to be displayed is fed as an analog signal, and after converted into a digital signal by an A/D converter 1, the signal is subjected to processings by a processor 2 such as gamma control of the display data voltage to adjust the electro-optical response (i.e., V-T curve) of liquid crystals, scaling for format conversion of the screen, etc. The signal thus processed is converted back into an analog signal by a D/A converter 3, and further converted into multiphase parallel signals by a plurality of sample-and-hold circuits 4. After that, with the frequency lowered to xe2x80x9c1/(the number of phases),xe2x80x9d the signals are supplied to a data driver of an LCD panel through linear amplifiers (not shown).
In the data driver, these and other data signals are sequentially held in capacitances of the source bus line according to the opening/closing of an analog switch (not shown) controlled by an output of a horizontal scanning circuit. Then, these data signals held by the source bus line are transferred to the capacitances of individual pixels connected to the source bus line via the TFTs until one horizontal scanning period terminates. After one horizontal scanning period has terminated, the data signals are held in the capacitances of the pixels.
On the other hand, in the driving operation of liquid crystals, for prevention of an orientation film and liquid crystals from deterioration due to electrochemical reaction as well as prevention of sticking or persistence of image, it is necessary to use an alternating voltage as the voltage applied to the liquid crystals. Therefore, as shown in FIG. 17B, the polarity of the video data signal is alternated every frame so that the AC driving is effectuated. As a result, a signal voltage whose polarity is alternately switched over every frame is applied across a pixel electrode, which has potentials determined by written data, and a counter electrode, whose potential is set to the intermediate potential between the potentials of the pixel electrode.
Liquid crystals respond to root mean square voltages. Thus, if the alternately positive and negative voltage has a completely symmetrical waveform, the resulting optical response occurs at a frequency at which the frame is switched (i.e., frame frequency). However, when the waveform is asymmetrical, even a little, there would arise a sub-harmonic component whose frequency is xc2xd of the frame frequency. Further, because a TFT has a characteristic that is not completely symmetrical in the positive and negative senses, there would also arise an offset of the DC potential due to feedthrough by switching. Thus, the potential of the counter electrode is set so as to counterbalance any effects of these. However, even if the waveform is adjusted so as to be completely symmetrical in polarity to one data voltage, it is extremely difficult to achieve a completely symmetrical waveform for all data voltages, because of the nonlinearity of capacitances of the liquid crystals and TFTs and/or the asymmetry in polarity and offsets of the gain of a linear amplifier circuit. Moreover, even if the waveform can be made completely symmetrical, the waveform may change with time and shift, resulting in an asymmetrical one.
Generally, the frame frequency is 60 Hz-85 Hz, and its secondary sub-harmonic frequency component, which has a frequency of 30 Hz-43 Hz, is observed as a flicker to human eyes, causing the display quality to be considerably impaired. To avoid this, it has conventionally been practiced to exert the so-called line inversion drive that the frequency at which liquid crystals blink is artificially doubled as shown in FIG. 17A so as to make the flicker indiscernible.
However, the small-sized active-matrix-driven LCD device used in conventional projection systems has the following problems. That is, in the drive method for the LCD device (scan line inversion drive in the example shown in FIG. 17A), voltages of opposite polarities are applied to adjoining pixel electrodes in order to avoid the flicker. Due to this, at electrode edge of a pixel electrode sandwiched by opposite-polarity voltages, a uniform electric field between the pixel electrode and the counter electrode (hereinafter, referred to as a xe2x80x9clongitudinal electric fieldxe2x80x9d) is disturbed, causing a component of an electric field in a transverse direction (hereinafter, referred to as a xe2x80x9ctransverse electric fieldxe2x80x9d) to arise. Therefore, for example, in the TN (Twisted Nematic) mode, there arises an inverted tilt region in response to a transverse electric field and depending on a pre-tilt. As a result, in the normally white mode in which the polarizer is set cross-Nicol, at and around a pixel electrode edge between the pixels with different polarity data, where the pre-tilt region and the inverted tilt region appear depending on the surface unevenness, pre-tilt angle, and the transverse electric field, there arise a region where light leakage occurs in the black display state as well as a region where the electro-optical response (V-T curve) of liquid crystals to display data voltages is shifted toward the higher voltage side. This would lower the contrast considerably. On the other hand, in the normally black mode in which the polarizer is set parallel-Nicol, the transmittance for the white level would lower by the above effects caused by the transverse electric field and, in addition to this, a high contrast as well as a neutral black are hard to realize because the same rotation of polarization axis or optical rotatory dispersion is not achieved over the entire visible range. Consequently, the practical display in the TN mode is limited to the normally white mode in which the polarizer is set cross-Nicol.
In order to obtain a sufficient display quality in the normally white mode, the region where the light-leakage occurs as well as the region where the V-T curve is shifted toward the higher voltage side need to be shielded from light. The region where the light-leakage occurs and the region where the V-T curve is shifted toward the higher voltage side extend over a generally constant distance from either pixel end, and thus, have noticeable effects particularly for small-sized pixels. In the conventional small-sized active-matrix-driven LCD device or the like used in the projection systems, the source line inversion drive was early adopted because the problems involved in drive would less occur. However, as the pixel size was scaled down more and more, the region to be shielded from light went relatively larger, which has led to a considerably lowered aperture ratio.
In the display section, as shown in FIG. 12, while only source bus lines 5 are provided vertically, gate bus lines 6 and common lines 7 for storage capacitors are provided laterally. Therefore, originally, the laterally-extending region that does not transmit light is larger than the vertically-extending region that does not transmit light. Thus, the scan line inversion drive, although having some problem in terms of drive, has come to be adopted for the purpose of utilizing, as a light-shield region, the laterally extending region that originally does not transmit light, and the light-shield region 8 is provided between pixels 9, 9 adjoining each other along the direction in which the source bus line 5 extends, so that decrease of the aperture ratio can be prevented.
However, in the conventional small-sized active-matrix-driven LCD device or the like used for the projection systems, if the pixel size becomes smaller, further light shield is needed for regions near upper and lower ends of the pixel electrodes where the contrast considerably lowers, which makes it extremely difficult to further raise the aperture ratio. A solution to such problems involved in the inversion drive is provided by a drive method eliminating spatial inversion display in the line inversion drive or the like (hereinafter, this drive method will be referred to as xe2x80x9cframe inversion drivexe2x80x9d) as shown in FIG. 17B.
However, in this frame inversion drive, optical response is made at a frequency at which the frame is switched over (i.e., frame frequency). In this connection, unfortunately, if the waveform is even slightly asymmetrical, there would arise sub-harmonic frequency components whose frequency is xc2xd of the frame frequency. As described before, the TFT characteristics are not completely symmetrical in polarity, which makes it extremely difficult to make a completely symmetrical waveform for all the data voltages, and moreover the waveform may be shifted due to changes with time, which causes the waveform to be asymmetrical.
Generally, the frame frequency is 60 Hz-85 Hz, its secondary sub-harmonic frequency component being 30 Hz-43 Hz. This secondary sub-harmonic frequency component is observed as a flicker, causing the display quality to be considerably impaired. To avoid this, a method for preventing the flicker by increasing the frame frequency about twice is disclosed in Japanese Patent Laid-Open Publication HEI 9-204159.
However, in the small-sized active-matrix-driven LCD device used in conventional projection systems, the driver monolithic structure using polysilicon TFTs has been adopted in order to avoid extremely reducing the pitch of connections of driver LSIs as described before. Polysilicon TFTs are considerably inferior in characteristics to single crystal silicon transistors, thus having limitations in high-speed operation. Therefore, in a conventional small-sized active-matrix-driven LCD device used in projection systems, the signal is converted into multiphase parallel signals by analog sample-and-hold circuits 4 as shown in FIG. 13, with the operating frequency lowered, and the signals are supplied to the data driver on the LCD panel. For example, when XGA (eXtended video Graphics Array) display is performed, the video signal is divided into as many as 12 phases per LCD device and the operating speed of the source data signal of XGA is dropped to {fraction (1/12)} by the data driver on the LCD panel side.
Accordingly, in order to prevent the flicker by raising the frame frequency twice in this state, it is necessary to divide the video signal into 24 (=12xc3x972) phases so as to obtain an usual video transfer rate. As a consequence, disadvantageously, not only the scale of external circuit is increased, with a cost increase incurred, but also the number of connections of input terminals to the LCD panel is increased and the scan driver is complicated, resulting in a decrease of yield. Further, in the case where the frame inversion drive is performed, since a parasitic capacitance exists between the source bus line and display pixels, reduction of the area of the storage capacitors to increase the aperture ratio would cause the parasitic capacitance to increase in ratio, which inevitably involves occurrences of crosstalk. This will invite a considerable deterioration of image quality.
In another respect, there are limitations in the voltage of display data (video signal) supplied from the data driver, from the viewpoints of transistors"" breakdown or withstand voltage and power consumption of the data driver. Therefore, when the horizontal line inversion drive is performed, a voltage loss is caused because of capacitances between pixels adjacent in the vertical direction and a higher drive voltage is required. That is, the drive voltage applied to the liquid crystals is insufficient. This causes a problem that a high contrast is hard to realize particularly in the normally white mode display.
Further, the horizontal line inversion drive has the following problems in the case where the data driver is so structured that the video signal is sequentially held in capacitors of the source bus line by analog switches controlled by output of the horizontal scanning circuit and where executed is the drive method that the video signal is divided into multiphase signals and then supplied in parallel with a lower frequency. That is, normally, the divided multiphase video signals are simultaneously sampled by analog switches. However, when capacitive coupling is present between right and left pixels, the hold potentials of adjoining pixels in a simultaneously-sampled pixel block and a next-sampled pixel block will vary, and the variations are observed as vertical stripes disadvantageously.
Therefore, an object of the present invention is to provide an LCD device capable of realizing a high contrast and a high aperture ratio at the same time and capable of high quality display.
In order to accomplish the above object, there is provided, according to a first aspect of the present invention, an active-matrix-driven LCD device in which a driver circuit and a display section are formed on one substrate and in which each of thin-film transistors included in the driver circuit and display section has an active layer made of a polysilicon that has been formed by enhancing crystal growth thereof, wherein the driver circuit operates to write data of a same polarity to all pixels of an entire one-frame screen but write data of different polarities to different frames adjoining each other on a time base at a frame frequency of about 100 Hz or more.
With this arrangement, each TFT used here has, as the active layer, a polysilicon formed by enhancing its crystal growth. Therefore, the TFTs have an electron mobility about twice higher than TFTs using normal polysilicon for the active layer, so that a frame inversion drive at a double rate of the normal frame frequency of about 60 Hz can be performed. As a result, there will occur no transverse electric fields between pixels adjoining each other along the source bus line, and this prevents the decrease of contrast due to occurrence of light leakage in the black display state in the normally white mode. Furthermore, even if the waveform of a data signal becomes asymmetrical due to asymmetry of data voltage, asymmetry of TFT characteristics, changes with time of data voltage or other reasons, and eventually a secondary sub-harmonic frequency component takes place, such a sub-harmonic frequency component will not be observed as the flicker thanks to the frame inversion drive at the double speed. Actually, the frame inversion drive at a frame frequency of about 100 Hz or higher will be able to produce effects similar to those produced by the frame inversion drive at the above double speed.
In this case, since the double-speed frame inversion drive is implemented by speeding up the operation of the driver circuit, an active-matrix-driven LCD device free from the flicker can be realized without any scale-up of external circuits, increase in connection counts of input terminals, complication of peripheral driver circuits, or increase in cost.
In other words, according to the invention, it is possible to dispense with a light-shield pattern to prevent the light leakage and thus to obtain a high aperture ratio. Thus, an LCD device which is high in aperture ratio and free from occurrence of contrast decrease and flicker, thus superior in display quality, can be provided.
In the LCD device, the driver circuit may comprise a data driver, and the LCD device may further comprise an electrically shielding means provided between a source bus line and pixel electrodes, the source bus line supplying data coming from the data driver to each of the pixel electrodes of the display section.
With this constitution, by the function of the electrical shielding means provided between the source bus line and the pixel electrodes, the effects of the capacitance between the source bus line and the pixel electrodes are reduced, so that vertical crosstalk can be prevented. Thus, the image quality is prevented from noticeable deterioration.
In the LCD device, the driver circuit may comprise a data driver which performs a dot sequential drive by which a plurality of parallelized data are simultaneously sampled.
With this arrangement, performing the frame inversion drive by the driver circuit can suppress a potential variation of a pixel electrode in a current simultaneously-sampled block which pixel electrode is in contact with a next sampled pixel block. Thus, the stripes on the screen are prevented.
In the LCD device, an array pitch of the pixels in the display section may be about 25 xcexcmxc3x9725 xcexcm or less.
With this constitution, in a high-definition LCD device in which the array pitch of pixels is about 25 xcexcmxc3x9725 xcexcm or less, the aperture ratio of pixels is enhanced, so that a high-quality image with high contrast and no flicker is displayed. Accordingly, the present invention realizes a high-quality small-sized active-matrix-driven LCD device of the driver monolithic type usable for projection systems.
In one embodiment, the parallelization of the data is performed so that twelve parallelized data are obtained. That is, the parallelization is performed in such a way that a dot clock of an original data signal made to be displayed by the data driver may be set 12 MHz or more per data.
With this arrangement, the dot clock of the normal speed data signal becomes about twice a normal one (6.25 MHz) and, in the XGA display, a double-speed drive of 12-phase development is performed.
There is provided, according to a second aspect of the invention, An active-matrix-driven LCD device having a driver circuit and a display section, wherein
the driver circuit operates to write data of a same polarity to all pixels of an entire one-frame screen but write data of different polarities to different frames adjoining each other on a time base, and
a capacitance is provided between pixel electrodes adjoining each other along a source bus line in the display section, said capacitance including an inter-electrode capacitance between the adjoining pixel electrodes and/or a capacitance given by an overlap of an electrically conductive light-shield layer with the adjoining pixel electrodes, said conductive light-shield layer being connected to a drain of a thin film transistor and disposed under the adjoining pixel electrodes with an insulating film interposed therebetween.
In this constitution, when the frame inversion drive is performed, a voltage resulting from adding a voltage corresponding to the capacitance between pixel electrodes adjoining each other along the extending direction of the source bus line to the voltage of data supplied from the driver circuit is applied to the pixel electrodes in the display section. Thus, the same contrast as in conventional LCD devices can be obtained with data of a lower voltage than in the conventional LCD devices, or else a sufficient drive voltage is applied to liquid crystals so that a high contrast can be obtained without sacrificing the aperture ratio of each pixel.
Consequently, according to this invention, in the normally white mode display, in which the drive voltage applied to liquid crystals becomes insufficient because of restrictions imposed on the level of data signals from the viewpoints of breakdown voltage of TFTs and power consumption, the insufficient drive voltage is compensated and a high contrast is obtained.
If a space between the pixel electrodes adjoining each other in a direction in which the source bus line extends is about 15% or less of a length along the source bus line of the pixel electrodes, a coupling capacitance that allows a sufficient voltage to be added to the voltage applied to the pixel electrodes is obtainable. Thus, a high contrast can be obtained without sacrificing the aperture ratio of each pixel, by the simple method of just adjusting the space between the pixel electrodes.
In one embodiment, the capacitance between pixel electrodes adjoining each other in a direction in which the source bus line extends is within a range of not less than 0.5% but not more than 10% of a storage capacitance associated with the pixel electrodes including a parasitic capacitance.
With this constitution, since the capacitance between pixel electrodes adjoining each other in the direction along which the source bus line extends is not less than 0.5% of the storage capacitance, the capacitance can sufficiently provide a voltage to be added to the voltage applied to the pixel electrodes. Further, since the capacitance between pixel electrodes is not more than 10% of the storage capacitance, variations in the capacitance between the pixel electrodes hardly affect the added voltage. Thus, a stable, sufficient voltage is added to the voltage applied to the pixel electrodes so that a high contrast can be obtained without sacrificing the aperture ratio of each pixel.